I. Field
The present invention relates generally to data communication, and more specifically to data transmission in a multi-carrier multiple-access communication system.
II. Background
A multiple-access system can concurrently support communication for multiple terminals on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. Multiple terminals may simultaneously transmit data on the reverse link and/or receive data on the forward link. This may be achieved by multiplexing the multiple data transmissions on each link to be orthogonal to one another in time, frequency, and/or code domain. Complete orthogonality is typically not achieved in most instances due to various factors such as channel conditions, receiver imperfections, and so on. Nevertheless, the orthogonal multiplexing ensures that the data transmission for each terminal minimally interferes with the data transmissions for the other terminals.
A multi-carrier communication system utilizes multiple carriers for data transmission. The multiple carriers may be provided by orthogonal frequency division multiplexing (OFDM), discrete multi tone (DMT), some other multi-carrier modulation techniques, or some other construct. OFDM effectively partitions the overall system bandwidth into multiple (K) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, frequency channels, and so on. Each subband is associated with a respective sub-carrier that may be modulated with data.
An orthogonal frequency division multiple access (OFDMA) system is a multi-access system that utilizes OFDM. An OFDMA system may use time and/or frequency division multiplexing to achieve orthogonality among multiple data transmissions for multiple terminals. For example, different terminals may be allocated different subbands, and the data transmission for each terminal may be sent on the subband(s) allocated to the terminal. By using disjoint or non-overlapping subbands for different terminals, interference among the multiple terminals may be avoided or reduced, and improved performance may be achieved.
The number of subbands available for data transmission is limited (to K) by the OFDM structure used for the OFDMA system. The limited number of subbands places an upper limit on the number of terminals that may transmit simultaneously without interfering one another. In certain instances, it may be desirable to allow more terminals to transmit simultaneously, e.g., to better utilize the available system capacity. There is therefore a need in the art for techniques to concurrently support more terminals in an OFDMA system.